Cold-curing silicone rubber materials, also referred to as “RTV” (“Raumtemperaturvernetzende”=cross-linking at room temperature) silicone rubber materials, have been known for quite some time as custom-designed materials which have elastic properties. They are used, generally, as sealants or adhesives for glass, porcelain, ceramics, stone, plastics, metals, wood, etc., in applications like joint filling and sealing compounds in construction and sanitary installations, or as coating agents, e.g. in the electronics industry (Römpp Chemie Lexikon, CD ROM, version 2.0, ed. J. Falbe, Thieme-Verlag, Stuttgart 1999; and Ullmanns Enzyklopädie der Technischen Chemie, 4th edition, ed. E. Bartholome, Verlag Chemie, Weinheim 1982, vol. 21, p. 511 et seq.). Use is made especially of single-component RTV silicone rubber materials (RTV-1), for example, plastically mouldable mixtures made of α,ω-dihydroxy-polyorganosiloxanes and appropriate so-called hardeners, or cross-linking agents, respectively, which are suitable for storing under exclusion of moisture but polymerize under the influence of water or humidity of the air at room temperature.
Preferably, various polyfunctional hardeners, e.g. tri- and/or tetrafunctional hardeners, are used together with various polyorganosiloxanes which either are difunctional or carry more functional groups, depending on the desired polymerization rate as well as the desired chemical and physical properties of the polymerization product, e.g. the desired degree of cross-linking, solvent resistance, etc. The most frequently chosen difunctional polyorganosiloxanes are α,ω-dihydroxy-polyorganosiloxanes. Polymerization, as a rule, takes place by condensation of the SiOH groups which are formed as intermediates by the hydrolysis of suitable hydrolyzable SiX groups of the hardeners. Based on the leaving groups (HX) released by the hydrolysis, a distinction is made with RTV-1 silicone rubber materials between acid systems (HX=acids, e.g. acetic acid), basic systems (e.g. HX=amines), and neutral systems (e.g. HX=alcohol or oxime). Since, when cross-linking, both acid and basic RTV-1 rubber materials release aggressive compounds which have the potential, for example, of corroding or decomposing metal, stone, or mortar, modern RTV-1 silicone rubber materials often are prepared with neutral cross-linking systems. Use is made, for example, of neutral cross-linking alkoxysilane hardeners which are based on methanol and ethanol as leaving groups. Commercially available alkoxy systems, however, are problematic as regards their storage stability and adherence of the polymerized rubber materials. That is why oximosilane hardeners are increasingly used which hydrolyze with release of an alkanone oxime. Especially hardeners which hydrolyze with release of butan-2-one oxime (or methyl-ethyl-ketoxime, MEKO, respectively) are often used at present.
Yet butan-2-one oxime may cause cancer, as has been discovered recently. Therefore, any further use of compounds releasing butan-2-one oxime, on principle, forbids itself for health reasons. That is why, since 2004, butan-2-one oxime must be labelled by the R phrase (risk phrase) “R40” (“suspected of having cancerogenic effects”). As a consequence, also silicone rubber materials must be thus labelled if they contain free butan-2-one oxime in a concentration above a certain threshold. The labelling requirement particularly includes silicone rubber materials like, for example, the ones contained in sealant cartridges, unless the free content of butan-2-one oxime is less than 1% (cf. “mixing rule” of the preparation directive, Directive 2006/8/EG of the Commission of Jan. 23, 2006, published in the Official Journal of the European Union of Jan. 24, 2006).
Practically all of the hardeners mentioned above suffer from another disadvantage, also under health aspects, namely that the compounds released during cross-linking smell awful, sometimes extremely awful, which causes great discomfort, especially, when working with them in closed spaces.
It is important for the formulation of the silicone rubber material that the hardener is liquid at room temperature, and preferably even at distinctly lower temperatures so that its handling will be easy and reliable and it will mix homogenously with the silicone rubber mass proper or the starting materials thereof, respectively. If the liquid state of the hardener is maintained even after transportation at minus temperatures during the winter that saves expenditure in terms of time and energy for melting it at the time of formulation.
Moreover, the hardener should accomplish the most complete polymerization possible of the rubber material in order to avoid subsequent “bleeding” of incompletely reacted starting substances etc. Finally, the polymerization product should be transparent and clear, respectively, when the curing is completed.